1. Field of the Invention
The present invention relates to a light emitting layer including molten salt and to an organic electroluminescence device including the light emitting layer and more particularly, to a light emitting layer including molten salt which forms a field induction charge separation layer when operated and thus, improves carrier injection, thereby providing improved light emitting efficiency, and to an organic electroluminescence device including the light emitting layer, having a low operating voltage and a long lifespan.
2. Description of the Related Art
Organic electroluminescent (EL) devices, which are active display devices, use the recombination of electrons and holes in a fluorescent or phosphorescent organic compound thin layer (hereinafter, referred to as ‘organic layer’) to emit light when current is applied thereto. Organic electroluminescent devices are lightweight, have wide viewing angles, produce high-quality images, and can be manufactured using simple processes. Organic electroluminescent devices also can produce moving images with high color purity while having low consumption power and low voltage. Accordingly, organic electroluminescent devices are suitable for portable electronic applications.
In general, an organic electroluminescent device includes an anode, a hole transport layer, an emission layer, an electron transport layer, and a cathode sequentially stacked on a substrate. The hole transport layer, the light emitting layer, and the electron transport layer are organic layers formed of organic compounds. The organic electroluminescent device may operate as follows. When a voltage is applied between the anode and the cathode, holes emitted by the anode move to the light emitting layer via the hole transport layer. Electrons are emitted by the cathode and move to the light emitting layer via the electron transport layer. In the light emitting layer, the carriers recombine to produce excitons. The excitons radiatively decay, emitting light corresponding to a band gap of the material used to form the light emitting layer.
Materials that can be used for forming the light emitting layer of the organic electroluminescent device are divided, according to the emission mechanism, into fluorescent materials using singlet excitons and phosphorescent materials using triplet-state excitons. The light emitting layer is formed by doping such fluorescent materials or phosphorescent materials themselves or by forming such fluorescent materials or phosphorescent materials on appropriate host materials. When electrons are excited, singlet excitons and triplet excitons are generated in a host in the generation ratio of 1:3 (Baldo, et al., Phys. Rev. B, 1999, 60, 14422).
When fluorescent materials are used to form the light emitting layer in the organic electroluminescent device, triplet excitons that are generated in the host cannot be used. However, when phosphorescent materials are used to form the light emitting layer, both singlet excitons and triplet excitons can be used, and thus, an internal quantum efficiency of 100% can be obtained (see Baldo et al., Nature, Vol. 395, 151-154, 1998). Accordingly, the use of phosphorescent materials brings higher light emitting efficiency than use of fluorescent materials.
However, although phosphorescent materials are used to improve light emitting efficiency, sufficient level of light emitting efficiency required in light emitting devices is not yet available. Accordingly, various methods to improve light emitting efficiency have been devised. For example, a method improving charge transport capacity using specific polymer materials optimizes the forming process of activated molecules performed by combining holes and electrons and thus, light emitting points are uniformly dispersed, thereby improving light emitting efficiency. A method to dispose a charge generation layer in a light emitting layer induces multiple wavelength light emission, instead of single wavelength light emission, and thus, light emitting efficiency can also be improved. In addition, electrical and physical properties between metals and organic layers are improved to control interfacial property and thus, light emitting efficiency can be improved. However, such methods have complicated processes, resulting in high costs and thus, sufficient level of light emitting efficiency required in light emitting devices can not be provided. Therefore, improvement of light emitting efficiency is still required.